CMP is a combination of chemical reaction and mechanical buffing. A conventional CMP system includes a polishing head with a retaining ring that holds and rotates a substrate (also referred to interchangeably as a wafer) against a pad surface rotating in the opposite direction or same direction. The pad can be made of cast and sliced polyurethane (or other polymers) with a filler or a urethane coated felt.
During rotation of the substrate against the pad, a slurry of silica (and/or other abrasives) suspended in a mild etchant, such as potassium or ammonium hydroxide, is dispensed onto the pad. The combination of chemical reaction from the slurry and mechanical buffing from the pad removes vertical inconsistencies on the surface of the substrate, thereby forming an extremely flat surface.
However, in CMP systems using retaining rings, uniform polishing of the substrate surface from the center of the substrate to the edge of the substrate is difficult to achieve. In particular, during CMP, applied down forces will cause the pad material to deform at the edge of the substrate, which leads to increased removal at the edge of the substrate and therefore a non-flat surface.
FIG. 1A is a block diagram illustrating a cross section of a prior art retaining ring, substrate and pad wherein uniform polishing of the substrate is not achievable. Retaining ring 30 is cylindrical in shape and holds substrate 40 in place during CMP. The retaining ring 30 includes smooth lower surface 32 for contacting a polishing pad during CMP, an inner surface 34 for retaining a substrate, an outer surface 36, a top surface 38 and a topper surface 50 that essentially caps the hollow region of the retaining ring 30. During CMP, frictional force between the substrate surface and a polishing pad 10 will push substrate 40 towards the trailing edge 45 of the retaining ring. The retaining ring 30, in addition to functioning to hold the substrate 40 in place, functions to press the pad 110 in a lateral motion during the polishing processes. The force on the inner surface 34 of the retaining ring 30 at the trailing edge 45 generates increased pressure on the pad 110 and causes the pad 110 to “flow” and be deformed toward the edge of the substrate, which leads to fast edge removal.
Therefore, a system and method for reducing edge removal and generating a more uniformly flat surface are needed.